As a new type of anti-seepage material, geomembrane plays a good role in the anti-seepage engineering of earth-rock dams. It has the advantages of strong impermeability, good frost resistance, no influence of seasonal climate, convenient construction, and saving engineering costs. Article This paper focuses on the anti-seepage application of geomembrane in earth-rock dam engineering and expounds on the problems that need to be paid attention to and solved in engineering.
(1) At present, due to construction quality problems, insufficient anti-seepage technology, inadequate maintenance and management, and uneven settlement of the dam body, the existing earth-rock dams have prominent problems of leakage and cracks, resulting in water accumulation behind the dam, etc. threaten the safety of the dam body, to prevent the occurrence or deterioration of the problem, corresponding engineering measures must be carried out.
(2) More anti-seepage measures are now used for anti-seepage wall anti-seepage, grouting anti-seepage, concrete anti-seepage, geomembrane anti-seepage, and other anti-seepage means. Geomembrane anti-seepage structure is an advanced technology in modern water conservancy projects, has a good anti-seepage effect, the construction is simple and the cost is low.
2 Characteristics of geomembrane
(1) Geomembrane is a kind of polymer synthetic material. It is used in the anti-seepage function of water conservancy construction: polyethylene (PE), polyvinyl chloride (PVC) and chlorinated polyethylene (CPE), etc. Geotechnical The physical, chemical, and hydraulic characteristics of the membrane can meet the requirements of hydraulic engineering. The thickness of the geomembrane used in engineering is in the range of 0.5-1.5mm, and the width of the geomembrane produced in a unified range is 2-4m. In some special, The width can be increased in the project.
(2) The geomembrane is an impermeable material, and the permeability coefficient is known from Darcy’s law
A geomembrane with K<10-14cm/s is considered impermeable. However, in specific engineering implementation, as long as the permeability coefficient K<10-13～10-15cm/s and the quality of the material do not have any problem with the geomembrane. Meet the needs of the project. The K value of the geomembrane is not fixed, and its size changes according to the value of the positive pressure on the membrane in the project, which generally shows a proportional relationship. To prevent the geomembrane from interacting with other materials, under pressure The geomembrane is damaged and loses its function when the soil particles in the contact layer are large or the contact layer is coarse. Thus, it is necessary to take corresponding anti-leakage experiments under certain conditions.
(3) The geomembrane as the main body of the anti-seepage structure often has weak links in the particle gaps of the support layer due to structural reasons, and it is easy to be crushed and broken down under a large working water pressure. Generally, the water pressure resistance of the geomembrane There is a positive correlation between the performance and the particle size distribution of the support layer material.
(4) The friction characteristics of the geomembrane and other materials in the protective layer or support layer are also
It is an important design index. Because of its smooth surface, the friction angle between it and other materials is smaller than that of soil. There is a danger of sliding along the interface when it is laid on a slope. Geomembrane and composite geomembrane with rough surfaces can be used. Or use the step-type laying method to solve it.
3 Geomembrane anti-seepage design
3.1 Arrangement and structure of geomembrane
3.1.1 According to the position of the geomembrane in the dam body, the anti-seepage structure of the geomembrane can be divided into two types: the anti-seepage inclined wall and the anti-seepage core wall. The geomembrane inclined wall is generally set on the upstream side of the dam, and the upstream slope is Almost parallel, the laying methods generally include straight laying of the thin protective layer, straight laying of a thick protective layer, and zigzag laying; geomembrane core wall is generally set in the middle of the dam body, or on the upstream side of the downstream pile of earth-rock mixed dam. In general In water conservancy projects, the anti-seepage structure of a geomembrane consists of three parts: geomembrane (film layer), support layer, and protective layer on the membrane, as shown in Figure 1.
3.1.2 The role of the support layer is to make the geomembrane stressed and free from local concentrated stress damage. The support layer is generally composed of a cushion layer and a transition layer. The support layer can generally adopt a traditional sandstone filter structure or can use Geotextile and sand and gravel mixed structure. The selection of cushion material is determined by the properties of the geomembrane. When using geomembrane, generally choose gravel, pebbles, or gravel with a particle size of less than 0.5cm as the cushion layer. When the composite geomembrane structure is used, since the geotextile has good wear resistance and plays a certain protective role, gravel or gravel with a particle size of less than 4cm can be used as a cushion.
3.1.3 The protective layer on the membrane plays the role of protecting the anti-seepage body. It is composed of the surface layer and the upper cushion layer. The protective layer can prevent factors such as ultraviolet radiation, water flow hitting and scouring, man-made damage, temperature rise and fall, and water pressure under the membrane. Hazards. Commonly used surface layers include prefabricated concrete slabs, cast-in-place concrete slabs, reinforced mesh or barbed wire concrete slabs, dry block stones, mortar block stones, etc. The choice of the cushion is determined by the type of geomembrane and surface layer.
3.2 Thickness of geomembrane
The geomembrane is the main body of the anti-seepage structure, and its thickness should be determined according to the requirements of anti-seepage and strength. The thickness of the geomembrane for low-head reservoirs is generally 0.5mm, and it can also be determined by the film theory formula, Giroud formula, and empirical formula, the reference values of allowable tensile stress and elastic modulus of polyethylene film are shown in Table 1.
3.2.1 Soviet empirical formula
3.3 Overall anti-seepage check of geomembrane anti-seepage structure
In order to ensure that the geomembrane can achieve the anti-seepage effect expected by the design and ensure the safe and efficient operation of the dam body, the following points should be done to check the anti-seepage of the geomembrane:
(1) Control the permeable water volume of the geomembrane to ensure that the permeated water volume is within the safe water volume and meets the design specification requirements.
(2) Control the design thickness of the geomembrane, even in the case of a small amount of water penetration, to avoid damage to the structure behind the membrane, seepage deformation, or take relevant engineering measures for the stability of the structure behind the membrane.
(3) Control the displacement of the dam body to be greater than the amount of seepage water, and keep the drainage of the structure behind the membrane unimpeded so that the seepage water can be removed in time.
3.4 Stability analysis of geomembrane as anti-seepage inclined wall
3.4.1 Protective layer with good water permeability
Rockfill, dry block stone slope protection, slope protection composed of concrete slabs and gravel materials, and prefabricated concrete slabs are all slope protection with good water permeability. In this case, when the water level of the reservoir falls, the soaked surface in the protective layer will also The safety factor can be calculated according to the formula
3.4.2 Protective layer with poor water permeability
When the protective layer on the membrane has poor water permeability or is impermeable due to design problems, and the drop in reservoir water level has no effect on the wetting surface in the protective layer, the reverse osmosis water pressure will affect the stability of the dam body, and the safety can be calculated according to the following formula coefficient
4 Geomembrane Construction
Since the geomembrane is a thin film material, it is damaged under high pressure. During the laying process, attention must be paid to the integrity of the geomembrane, so as not to damage the structure of the geomembrane and affect its anti-seepage effect. According to the corresponding specifications and requirements, it is necessary to First, treat the underlying layer to remove impurities, and at the same time ensure the tightness of the geomembrane to avoid air bubbles, wrinkles, or excessive tension. Before the construction of the protective layer, the geomembrane must be prevented from being blown by the wind Thus, it is best to construct the protective layer while laying the geomembrane.
For geomembrane splicing with limited width, there are currently two methods used at home and abroad, on-site welding and bonding. Among them, electrothermal wedge welding is the most widely used welding method, and hot-melt extrusion and high-temperature gas welding are also welding methods. The welding method needs to pay attention to whether the membrane and the membrane can be bonded under the influence of high temperature without producing a weak section that will cause the membrane to break and affect the anti-seepage effect. Compared with the welding method, the bonding method is complicated. The requirements for the construction environment and quality control are high, and it is generally suitable for occasions with gentle slopes. For steep slope construction, the bonding is compacted with a wooden hammer. However, the bonding method has advantages in defect repair.
Anchoring is one of the most important links in geomembrane anti-seepage construction. To ensure that the geomembrane anti-seepage structure forms a complete anti-seepage system with the dam body or other anti-seepage structures, and to ensure that the geomembrane plays an anti-seepage role, the geomembrane must be well prepared. Anchoring work. The upper edge of the geomembrane must be embedded in the anchoring groove and ensure a certain embedded length, or be connected with the wave wall at the top of the dam, and the lower edge must be embedded in the bottom of the dam to prevent leakage of the foundation.
Geomembrane can solve the leakage problem of the dam body and has good economic benefits. controlling product quality, controlling construction technology, and paying attention to effect verification will have a wider application prospect.